The invention relates to an anchoring device for anchoring a rod in bones or vertebrae which can be used with at least two rods having a different diameter.
For stabilizing the spinal column, bone anchoring devices are known which comprise a shaft to be anchored in the bone and a head to be connected to a rod. Usually, a rod connects several bone anchoring devices. Depending on the medical indication and the region of the spine where the stabilization is required rods with different diameters are needed. The diameter of the rods ranges from 3 mm to more than 6 mm. Generally, the diameter of the rod which is used in the lower part of the spine is larger than the diameter of the rod which is used in the upper part of the spine. For example, as shown in FIG. 1 in the cervical-thoracic region of the spine rods with a diameter of 3 mm to 3.5 mm, in the transitional zones between the cervical-thoracic and the thoracic-lumbar region rods with a diameter of 3.5 mm to 4.5 mm, in the thoracic-lumbar region rods of usually 4.5 mm to 5.5 mm and the lumbar-sacral region rods with a diameter of 5 mm to 6.35 mm are necessary.
Typical bone anchoring devices have a recess for receiving a rod. The diameter of the recess may be slightly larger than the diameter of the rod in order to account for small variations in manufacturing tolerances. The tolerance may allow rod diameter variation of about 2%. However, the noted tolerance is not sufficient to allow the use of different diameter rods as discussed above for different spinal surgery applications.
For each diameter of the rod, specific bone anchoring devices, such as pedicle screws, are required. They differ from each other in particular by the size of the recess into which the rod is inserted. The provision of having different bone anchoring devices available for surgery increases the costs and renders spinal surgery more complicated for surgeon.
U.S. Pat. No. 5,873,878 discloses an anchoring member for attachment to a vertebra and for use with a first rod having a first diameter and a second rod having a second, smaller diameter. The anchoring member has an insert member which can be inserted into the head of the anchoring member so as to allow the insertion of a rod with a smaller diameter.
Based on the above, there is a need to provide an anchoring device which can be used with several rods having a different diameter.